Okay! I’m sorry, I don’t know what the root of the problem is.

Have you tried using the Jetpack App for posting images directly from your phone? Alternatively, if you want to use a Shortcut action, you can either use Micropub (see https://cagrimmett.com/development/2023/02/12/ios-shortcut-actions-for-micropub-posting/) or the regular WordPress REST API (https://cagrimmett.com/2023/11/09/apple-shortcuts-for-posting-to-wordpress-via-the-rest-api-and-xml-rpc-metaweblog-api/)